NZ Weather Forum

Seeing as how there has been some chat about this in the General July eather thread already, and we are only 4 days out, I thought it best to create a new thread.

Models have been indicating a low crossing the country and rapidly deepening of the coast of Canterbury on Thursday night. Almost to the point of "Weather Bomb" status.. (dare I say it, currently forecast to drop about 20hpa in 24 hours I believe). Wind, rain and even snow will be an issue based on current models with a Warm Advection element present. And then a cold SW flow left behind for the weekend with showers.
Obviously models will change between now and then, but with this showing up for the last 3 days. its fair to say something interesting will play out.

Yes 976hpa on Friday morning is an intense low that's for sure!

Bradley wrote:Yes 976hpa on Friday morning is an intense low that's for sure!

Gets down to 974hpa by Friday evening!!

WOAH, latest metvuw run is through... that is nasty for Southland!! Huge amounts of moisture there with the undercutting southerly! If that comes off there will be severe issues!

That looks incredibly nasty if it happens! Up to around 120mm from early Friday til late Saturday for us here if that eventuated

hozza95 wrote:That looks incredibly nasty if it happens! Up to around 120mm from early Friday til late Saturday for us here if that eventuated

It all depends on the positioning of the low. But you're correct... on current modelling it looks nasty for everyone south of ChCh.

Wow latest EC model run is off the charts with around 80-100mm for mid-Canterbury! Not that interested in low level snow though..

Bradley wrote:Wow latest EC model run is off the charts with around 80-100mm for mid-Canterbury! Not that interested in low level snow though..

I'm not seeing a low level snow event either Bradley. Not going by the latest data. A heap of rain though! A lot if that will be snow on higher elevations .. Could be some heavy showers/ hail this afternoon though? As it looks like alittle upper cold trough is passing through Canterbury

Have you seen the latest EC forecast for Lake Tekapo though?? 110cm of snow over a 2 day period!!

Metservice now going for heavy snow above 400m in Canterbury on Friday, it's the lowest I have seen anyone at this stage forecast the snow to go which is unusual for the Metservice who are usually quite reserved with that sort of thing. They must see some warm advection scenario going on with that cold air pushing further northwards then some others are expecting...these scenarios can surprise alot, you just have to remember back to to the June 2006 event where they only forecast 15cm down to 300m asl the night before the event and we all know what happened on the Monday!!

WW are thinking it could be as low as 200-300m in their article they have just put up. Normally they are far more accurate than MS when it comes to snow events ( in my opinion anyway)

snowchaser01 wrote:

hozza95 wrote:That looks incredibly nasty if it happens! Up to around 120mm from early Friday til late Saturday for us here if that eventuated

It all depends on the positioning of the low. But you're correct... on current modelling it looks nasty for everyone south of ChCh.

Some models I've seen have the low move down a bit further to the west, sparing Chch/Banks Peninsula but hammering South Canterbury southwards.

Bradley wrote: They must see some warm advection scenario going on

It is a warm advection scenario. Its an overrunning scenario with moist E/SE winds aloft over running a relatively cold southerly near the surface. The moisture aloft originates from the subtropics.

For me this setup suggests snow down to 100m with significant accumulations above 300m

Metservice now lowering the snow level to 300m on Friday for Canterbury so places like Methven, Mayfield, Sheffield etc could be in the firing line!

Not so much Mayfield, its 270m asl

Richard wrote:Not so much Mayfield, its 270m asl

In practice, I've observed it does tend to fall to the Mayfield area when forecast to 300m.

30m meters is a bit like splitting hairs when forecasting snow anyway i would have thought.

melja wrote:30m meters is a bit like splitting hairs when forecasting snow anyway i would have thought.

Yes Tony has said that even 100m is nearly impossible to forecast when it comes to snow and certainly in a warm advection scenario

Bradley wrote:

melja wrote:30m meters is a bit like splitting hairs when forecasting snow anyway i would have thought.

Yes Tony has said that even 100m is nearly impossible to forecast when it comes to snow and certainly in a warm advection scenario

Indeed. I still dont see this getting below 500m. There is virtually no cold air coming in from the south. ECMWF forecast temps for Chch city are between 9 and 10 degrees during the day time on Friday. Lots of rain though.

The front page of this morning's Timaru Herald has a warning for heavy rain/snow for South Canterbury on Friday.

Tony, there is an advection element to this though? So models don't pick it up to the full extent? Thk levels are way up.. yes. But back in the 2006 event they were too.

snowchaser01 wrote:Tony, there is an advection element to this though? So models don't pick it up to the full extent? Thk levels are way up.. yes. But back in the 2006 event they were too.

I dont think its quite correct to say the models dont pick it it. They fully pick up the advective precipitation physics. If they didnt they would be useless in all parts of the world. What they sometimes dont know about is the temperatures in the lower layers in our more complex terrain (hills and valleys) which can sometimes be colder than the model realises (thats a resolution and initialisation issue). However, I'm not convinced that in this case that will be so, as I can't see any significantly cold air likely to be present.

TonyT wrote:

snowchaser01 wrote:Tony, there is an advection element to this though? So models don't pick it up to the full extent? Thk levels are way up.. yes. But back in the 2006 event they were too.

I dont think its quite correct to say the models dont pick it it. They fully pick up the advective precipitation physics. If they didnt they would be useless in all parts of the world. What they sometimes dont know about is the temperatures in the lower layers in our more complex terrain (hills and valleys) which can sometimes be colder than the model realises (thats a resolution and initialisation issue). However, I'm not convinced that in this case that will be so, as I can't see any significantly cold air likely to be present.

agreed. I too don't see the temperatures cold enough despite advective components. Latest low layer temps 925mb at 7-8C 850mb sits at 4-5C And mid layers at 700mb is only about -4 NOTE: "that's all going by Friday midday temperatures" I also understand the models don't pick everything up when it comes to warm advective events though aswel. Like you say Tony lots of rain!! 50mm plus fur the city

At the rate all the models have been keeping the colder air more south of Canterbury it's looking like 600m will be more the mark for heavy snow - unless of course something changes in the next 24-48 hours...